The current invention relates to a process for removing undesired components from waste materials. More specifically, the present invention presents a method for removing metallic and organic materials from basic aqueous solutions by means of co-precipitation with a stabilized gel, typically comprising aluminum hydroxide and a complexing agent such as EDTA.
Contamination resulting from diverse industries including manufacturing, power generation, mineral extraction, and nuclear weapons production operations has resulted in extensive environmental impacts to facilities and their surrounding environments. Contaminants include heavy metals, organics, and radionuclides occurring individually or as mixed waste. A need exists for means to recover these contaminants while assuring waste minimization.
Contaminants are often bound to surface substrates in pipes and reaction vessels or on the surface of soils and in groundwater. Soil and groundwater treatment often involves pump and treat methods or direct soil extraction and washing. Contaminants of concern in soils are bound to soil particle surfaces and can often be removed by solvent extraction.
Existing contaminant recovery processes typically use acid wash techniques which cause corrosion of metallic components, especially iron, when used for decontaminating facility piping. Moreover, acid wash is hampered in its application to soils because it quickly becomes saturated with carbonates, sulfates, and to a lesser extent, other common soil anions.
Existing extraction techniques typically use adsorbant or absorbant solids which bind metal ions on their surfaces or in their structures. For example, zeolites and charcoal filters are used to absorb radionuclides. In facility decontamination processes, organic reagents are commonly used to dissolve contaminants. The reagents in decontamination processes are typically used to deposit the contaminants on ion exchange resins. However, in any of these processes, the absorbing solids themselves become waste.
Contaminated waste solutions containing high concentrations of heavy metals or radionuclides and chelating agents such as EDTA are typically generated during the process of decontaminating metal surfaces or through soil washing. A number of methods have been previously described for treatment and disposal of these solutions.
Typically, hazardous components are recovered as solids from non-hazardous components. The solid components including radionuclides are sent to landfills with special restrictions for final disposal. Waste disposal restrictions are also placed on chelating agents such as EDTA because they can keep heavy metals or radionuclides in solution and enable them to migrate to drinking water supplies. Therefore, limits are often placed on disposal of chelants in disposal sites.
Therefore, a number of methods have been utilized which involve the separation of the chelating agent from the metals. Once separated, the contaminated chelating agent is either subjected to volume reduction techniques or destroyed. Destruction techniques include oxidation (e.g., ozone or permanganate), pyrolysis, electrolytic decomposition and others.
Volume reduction is subject to processing costs and disposal fees. Other methods typically require expensive equipment or tend to take long processing times such as ozonation.
In the prior art, metal hydroxides such as aluminum hydroxide form gels which are utilized in laboratories to scavenge contaminants. However, the hydroxide gels which form typically convert to a range of crystalline forms which precipitate. These crystalline precipitates are difficult to dissolve. The invention provides a phase transition gel, that is, a gel that can be readily dissolved and reprecipitated by simple pH adjustment.